Are your kids anything like mine? McDonald's always seemed to have some kind of magical, magnetic pull on my little ones. They couldn't see the dirty clothes on the floor of their bedrooms, but they could spot the Golden Arches from miles away, and they always wanted to stop for a Happy Meal and a romp on the playground equipment. From all the other families I've seen at McDonald's, I'm sure my crew is not unique. My children have grown out of the Happy Meal stage, but they still like their burgers and fries, and McDonald's is still a place they love to stop. Our family has always had good experiences there. That's why I was surprised while working on our Dirty Dining report when I heard about what happened last fall in Piqua, Ohio.

More than 120 people were sickened after eating ice cream at their local McDonald's. The health department says the restaurant's dairy mixture somehow was contaminated with staphylococcus, and a mechanical malfunction in the soft serve machine allowed the bacteria to grow. So many people became so ill, so quickly, the director of the local emergency room told me he at first thought there was some kind of bioterrorism incident in the town.

This food borne bug hits hard, and we talked to about a dozen people who will testify to that. Patrick Johnson is a big, strapping guy in his early 20s. And he said he had to have an ambulance crew carry him out of his house. The bug he picked up by drinking a milkshake laid him low. Hilda Taylor, a sweet 70-year-old who had some ice cream at the Piqua McDonald's, said she was so sick she thought she was going to die. Her kidneys shut down for a time, but she managed to rally and pull through. Nancy Smith says she had taken her grandchildren for an outing, and stopped at McDonald's to buy them a treat. Her grandson Darien had a milkshake, and she says he was violently ill an hour later. He became limp and listless, Smith says, and they rushed him to the hospital, scared to death. Then she got sick. She says she had just three sips of Darien's shake, but it was enough to make her very ill. She told us she was vomiting so violently, she thought she was going to have a heart attack.

She's now suing McDonald's, as are many of the other people who got sick in that incident. McDonald's responded that "this was an unfortunate, isolated case. According to the local health department’s investigation, this incident was caused by a naturally occurring organism commonly found in the human body. The health department determined that a mechanical malfunction of the ice cream/milk shake machine was also a contributing factor."

We have to put this in context. Remember, fast food restaurants serve a staggering 61 million meals safely every day, according to the National Restaurant Association. So your chances are getting sick are slim, but as you can see in the Piqua case, when it happens, it can be devastating. This time, it was a McDonald's. But other chains make the occasional mistake too. That's why we do months of research to find out which fast food chains do the best job of keeping things safe and clean. Knowing which chains have the best track records can help you keep your family from going through the kind of illness these people in Piqua suffered. But how do you rank the cleanliness and food handling records of the big chains? We thought it over, and came up with a new method -- we think Dateline is the first to use it to do a systematic evaluation of fast food cleanliness.

We thought the fairest way to rank the chains was to use the results of restaurant health inspections. So we first hired a survey company to pick 1,000 restaurants - 100 from each of the top ten chains. Then we collected a year's worth of inspection reports from those 100 locations, and analyze them for critical violations. Those are the violations inspectors say have the potential to make you sick. Since some states and counties inspect restaurants more often than other jurisdictions, we analyzed them for critical violations and then averaged the number of critical violations per routine inspection. We did the math, and came up with our Dirty Dining list.

Then we took our hidden cameras and went out to see for ourselves. We went along with a young, Nashville inspector named Yvonne Martinez. Martinez is the kind of person you want to have checking up on cleanliness. She didn't miss a thing when we were with her. She didn't nit pick, but rather, she showed us what was important in food safety, like holding food at the proper temperature. If you don't keep hot foods hot enough -- over 140 degrees -- that gives any bacteria in the food a chance to grow and make you sick when you eat that food. It's the same with cold food -- it has to be kept below 41 degrees. And hand washing. Our mothers all told us to wash our hands before we eat. But when you see just how sick you can get if you don't, and pick up a norovirus or something else, you certainly will remember next time.

We found some restaurant chains that seemed to be doing a very good job.

Taco Bell avoids problems by cooking its meat products at commercial kitchens, so employees in their restaurants do not have to cook. They just heat up the frozen meats, and serve. It seems to work, because Taco Bell did very well in our survey, and Inspector Martinez told us in her experience, Taco Bell does a good job training its employees.

All of the chains seem to have strong food safety systems in place. But as long as human beings are involved in the process, mistakes are going to happen. That's why the National Restaurant Association says it stresses the importance of employee training, and runs "Safe Serv" training sessions. Training and a vigilant management team help some of the chains minimize mistakes.

You can find out which chains did the best job in our survey by watching our Dirty Dining report.

Dateline wires the home of a volunteer, Jenny, from top to bottom with hidden cameras. Then she called repairmen to her house for a simple problem we created as a test with Jenny's pool. NBC's Chris Hansen reports.